The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with localized coils for medical imaging equipment to receive electromagnetic signals from resonating nuclei and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find utility in other magnetic resonance applications, such as exciting resonance, chemical analysis, well logging, and the like.
Heretofore, various types of coils have been positioned to receive electromagnetic signals for magnetic resonance imaging and spectroscopy, including whole body, body portion, and localized coils. The whole body and body portion receiving coils had standard sizes which were selected for readily receiving the patient's whole body or a selected body portion. Due to the standardized coil size and variable patient size, a significant void or empty region was commonly defined between the coil and the portion of the patient to be imaged.
Localized or surface coils were configured from rigid and flexible non-conductive sheets of plastic or nylon on which wire loops were mounted. Rigid flat coils were constructed in a variety of sizes to facilitate positioning adjacent the selected area of the patient to be imaged. When a flat coil was positioned adjacent a relatively flat area of the patient, the intervening air gap was relatively small as the air gap associated signal-to-noise degradation and aliasing. Flexible coils could be bent or wrapped around curved portions of the patient for a better, substantially air gap-free fit.
The wire or other conductors on the localized coils were commonly wound in circular loops on the surface of the plastic sheet. In use, the loops were positioned parallel to the surface of the patient. With a patient positioned along a z-axis magnetic field, coil loops disposed adjacent the patient's back were positioned in an x-z plane. An image plane transversely through the patient, e.g. an x-y plane, would normally intersect the surface coil at two points. Radio frequency magnetic resonance signals originating within this plane induced a like radio frequency current signal within coil segments at the intersection of the x-y plane, particularly segments that extend perpendicular to the x-y plane. However, because the coil was connected in a loop, the currents induced at opposite points were in the opposite direction around the loop and, hence, tended to cancel.
To receive signals from deeper within the patient, larger diameter loops were utilized. The depth of the coil's region of sensitivity has been adjusted by selecting more complex winding patterns, such as a planar arrangement of concentric loops through which current flows in different directions. However, the complex loop arrangements still had the high magnetic energy losses of the single loop.
At high frequencies, coils placed against the subject interacted strongly with the subject. The coils were most sensitive to the regions of the subject which were in immediate proximity to the coil segments. Increasing the complexity of the coil winding placed additional coil segments adjacent the subject, each of which was more sensitive to the immediately contiguous region. The more windings in the coil pattern, the more the signals from regions adjacent the subject surface dominated the signals from more remote regions. Other problems attributable to high sensitivity near the conductors were also aggravated.
A localized coil is provided which overcomes the above referenced problems and others by interacting with the subject in an optimal manner. In a transmit mode, it produces a more homogeneous field pattern within the subject and a receive mode provides an improved signal-to-noise ratio.